feat: add support for -c/--config to override individual config items (#1137)

This PR introduces support for `-c`/`--config` so users can override
individual config values on the command line using `--config
name=value`. Example:

```
codex --config model=o4-mini
```

Making it possible to set arbitrary config values on the command line
results in a more flexible configuration scheme and makes it easier to
provide single-line examples that can be copy-pasted from documentation.

Effectively, it means there are four levels of configuration for some
values:

- Default value (e.g., `model` currently defaults to `o4-mini`)
- Value in `config.toml` (e.g., user could override the default to be
`model = "o3"` in their `config.toml`)
- Specifying `-c` or `--config` to override `model` (e.g., user can
include `-c model=o3` in their list of args to Codex)
- If available, a config-specific flag can be used, which takes
precedence over `-c` (e.g., user can specify `--model o3` in their list
of args to Codex)

Now that it is possible to specify anything that could be configured in
`config.toml` on the command line using `-c`, we do not need to have a
custom flag for every possible config option (which can clutter the
output of `--help`). To that end, as part of this PR, we drop support
for the `--disable-response-storage` flag, as users can now specify `-c
disable_response_storage=true` to get the equivalent functionality.

Under the hood, this works by loading the `config.toml` into a
`toml::Value`. Then for each `key=value`, we create a small synthetic
TOML file with `value` so that we can run the TOML parser to get the
equivalent `toml::Value`. We then parse `key` to determine the point in
the original `toml::Value` to do the insert/replace. Once all of the
overrides from `-c` args have been applied, the `toml::Value` is
deserialized into a `ConfigToml` and then the `ConfigOverrides` are
applied, as before.

codex-rs/exec/src/cli.rs

@@ -1,5 +1,6 @@
 use clap::Parser;
 use clap::ValueEnum;
+use codex_common::CliConfigOverrides;
 use codex_common::SandboxPermissionOption;
 use std::path::PathBuf;
 
@@ -33,9 +34,8 @@ pub struct Cli {
     #[arg(long = "skip-git-repo-check", default_value_t = false)]
     pub skip_git_repo_check: bool,
 
-    /// Disable server‑side response storage (sends the full conversation context with every request)
-    #[arg(long = "disable-response-storage", default_value_t = false)]
-    pub disable_response_storage: bool,
+    #[clap(skip)]
+    pub config_overrides: CliConfigOverrides,
 
     /// Specifies color settings for use in the output.
     #[arg(long = "color", value_enum, default_value_t = Color::Auto)]

codex-rs/exec/src/lib.rs

@@ -34,10 +34,10 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
         sandbox,
         cwd,
         skip_git_repo_check,
-        disable_response_storage,
         color,
         last_message_file,
         prompt,
+        config_overrides,
     } = cli;
 
     let (stdout_with_ansi, stderr_with_ansi) = match color {
@@ -63,16 +63,20 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
         // the user for approval.
         approval_policy: Some(AskForApproval::Never),
         sandbox_policy,
-        disable_response_storage: if disable_response_storage {
-            Some(true)
-        } else {
-            None
-        },
         cwd: cwd.map(|p| p.canonicalize().unwrap_or(p)),
         model_provider: None,
         codex_linux_sandbox_exe,
     };
-    let config = Config::load_with_overrides(overrides)?;
+    // Parse `-c` overrides.
+    let cli_kv_overrides = match config_overrides.parse_overrides() {
+        Ok(v) => v,
+        Err(e) => {
+            eprintln!("Error parsing -c overrides: {e}");
+            std::process::exit(1);
+        }
+    };
+
+    let config = Config::load_with_cli_overrides(cli_kv_overrides, overrides)?;
     // Print the effective configuration so users can see what Codex is using.
     print_config_summary(&config, stdout_with_ansi);
 

codex-rs/exec/src/main.rs

@@ -10,13 +10,30 @@
 //! This allows us to ship a completely separate set of functionality as part
 //! of the `codex-exec` binary.
 use clap::Parser;
+use codex_common::CliConfigOverrides;
 use codex_exec::Cli;
 use codex_exec::run_main;
 
+#[derive(Parser, Debug)]
+struct TopCli {
+    #[clap(flatten)]
+    config_overrides: CliConfigOverrides,
+
+    #[clap(flatten)]
+    inner: Cli,
+}
+
 fn main() -> anyhow::Result<()> {
     codex_linux_sandbox::run_with_sandbox(|codex_linux_sandbox_exe| async move {
-        let cli = Cli::parse();
-        run_main(cli, codex_linux_sandbox_exe).await?;
+        let top_cli = TopCli::parse();
+        // Merge root-level overrides into inner CLI struct so downstream logic remains unchanged.
+        let mut inner = top_cli.inner;
+        inner
+            .config_overrides
+            .raw_overrides
+            .splice(0..0, top_cli.config_overrides.raw_overrides);
+
+        run_main(inner, codex_linux_sandbox_exe).await?;
         Ok(())
     })
 }